The first step towards protecting an RS-422 or RS-485
system from transients is understanding the nature of the
energy we are guarding against. Transient energy may come from
several sources, most typically environmental conditions or
induced by switching heavy inductive loads.
What does a surge look
like?
Surge
Specifications
While transients may not always
conform to industry specifications, both the Institute of
Electrical and Electronics Engineers (IEEE) and the
International Electrotechnical Commission (IEC) have developed
transient models for use in evaluating electrical and
electronic equipment for immunity to surges. These models can
offer some insight into the types of energy that must be
controlled to prevent system damage.
Both IEC 1000-4-5: 1995 “Surge Immunity Test” and IEEE
C62.41-1991 “IEEE Recommended Practice on Surge Voltages in
Low-Voltage AC Power Circuits” define a “1.2/50µs - 8/20µs
combination wave” surge which has a 1.2 µs voltage rise time
with a 50 µs decay across an open circuit. The specified
current waveform has an 8 µs rise time with a 20 µs decay into
a short circuit. Open circuit voltages levels from 1 to 6 kV
are commonly used in both the positive and negative
polarities, although, under some circumstances, voltages as
high as 20 kV may be applied. Figures 4.1 and 4.2 illustrate
the combination wave characteristics. In addition, IEEE C62.41
also specifies a 100 kHz “ring wave” test. The ring wave has a
0.5 µs rise time and a decaying oscillation at 100 kHz with
source impedance of 12 ohms as shown in Figure 4.3. Typical
amplitudes for the 100 kHz ring wave also range from 1 – 6
kV.
Figure 4.1 - Combination Wave Voltage Waveform
Figure 4.2 Combination Wave Current Waveform
Figure 4.3 100 kHz Ring Wave
Common Mode vs.
Differential Mode
Identifying the type of surges
that may threaten a system is an important part of selecting
the appropriate levels and methods of transient protection.
Since each of the conductors in a data cable travels through
the same physical space, it is reasonable to expect transients
caused by environmental or current switching to be “common
mode” that is, present on all data and ground conductors
within the data cable. In some installations, there may be
another source of unwanted energy to consider. If there are
high voltage cables running anywhere near the data cables, the
potential for a fault condition exists as a result of
insulation failures or inadvertent contact by an installer.
This type of surge could contact any number of conductors in
the data cable, presenting a “differential” surge to the data
equipment. Although the voltages and currents associated with
this type surge are much lower than the types of surges
modeled by ANSI or IEC, they have a particularly destructive
quality of their own. Instead of dissipating within several
milliseconds, they can exist in a steady state condition on
the data network.
Ground Is Not Equal To
Ground
Realizing that transient energy can be
high frequency in nature leads to some disturbing
observations. At frequencies of this magnitude, it is
difficult to make a low impedance electrical connection
between two points due to the inductance of the path between
them. Whether that path is several feet of cable or thousands
of feet of earth between grounding systems, during a transient
event there can be hundreds or thousands of volts potential
between different “grounds”. We can no longer assume that two
points connected by a wire will be at the same voltage
potential. To the system designer this means that although
RS-422/485 uses 5V differential signaling, a remote node may
see the 5V signal superimposed on a transient of hundreds or
thousands of volts with respect to that nodes local ground. It
is more intuitive to refer to what is commonly called “signal
ground” as a “signal reference”.
How do we connect system nodes knowing that these large
potential differences between grounds may exist? The first
step towards successful protection is to assure that each
device in the system is referenced to only one ground,
eliminating the path through the device for surge currents
searching for a return. There are two approaches to creating
this idyllic ground state. The first approach is to isolate
the data ground from the host device ground, this is typically
done with transformers or optical isolators as shown is Figure
4.4. The second approach is to tie each of the grounds on a
device together (typically power ground and data ground) with
a low impedance connection as shown in Figure 4.5. These two
techniques lead us to the two basic methods of transient
protection.
Figure 4.4 Isolated RS-485 Device
Figure 4.5 RS-485 Device with Signal Ground Connected to
Chassis Ground
Transient Protection
using Isolation
Isolation Theory
The most universal
approach to protecting against transients is to galvanically
isolate the data port from the host device circuitry. This
method separates the signal reference from any fixed ground.
Optical isolators, transformers and fiber optics are all
methods commonly used in many types of data networks to
isolate I/O circuitry from its host device. In RS-422 and
RS-485 applications, optical isolators are most common. An
optical isolator is an integrated circuit that converts the
electrical signal to light and back, eliminating electrical
continuity. With an isolated port, the entire isolated
circuitry floats to the level of the transient without
disrupting data communications. As long as the floating level
of the circuitry does not exceed the breakdown rating of the
isolators (typically 1000 - 2500 volts) the port will not be
damaged. This type of protection does not attempt to absorb or
shunt excess energy so it is not sensitive to the length of
the transient. Even continuous potential differences will not
harm isolated devices. It is important to note that isolators
work on common mode transients, they cannot protect against
large voltage differences between conductors of a data cable
such as those caused by short circuits between data and power
circuits.
Isolation
Devices
Optical isolation can be implemented in
a number of ways. If a conversion from RS-232 to RS-422 or
RS-485 is being made, optically isolated converters are
available. Optically isolated ISA bus serial cards can replace
existing ports in PC systems. For systems with existing RS-422
or RS-485 ports, an optically isolated repeater can be
installed. Examples of each of these type devices can be found
in the B&B Electronics Data Communications catalog.
Transient Protection
using Shunting
Shunting Theory
Creating one common
ground at the host device provides a safe place to divert
surge energy as well as a voltage reference to attach surge
suppression devices to. Shunting harmful currents to ground
before they reach the data port is the job of components such
as TVS (often referred to by the trade name Tranzorb), MOV or
gas discharge tubes. These devices all work by “clamping” at a
set voltage, once the clamp voltage has been exceeded, the
devices provide a low impedance connection between
terminals.
`Since this type of device diverts a large amount of energy,
it cannot tolerate very long duration or continuous
transients. Shunting devices are most often installed from
each data line to the local earth ground, and should be
selected to begin conducting current at a voltage as close as
possible above the systems normal communications levels. For
RS-422 and RS-485 systems, the voltage rating selected is
typically 6 - 8 volts. These devices typically add some
capacitive load to the data lines. This should be considered
when designing a system and can be compensated for by derating
the total line length to compensate for the added load.
Several hundred feet is usually adequate.
To apply these type products correctly they should be
installed as close to the port to be protected as possible,
and the user must provide an extremely low impedance
connection to the local earth ground of the unit being
protected. This ground connection is crucial to proper
operation of the shunting device. The ground connection should
be made with heavy gauge wire and kept as short as possible.
If the cable must be longer than one meter, copper strap or
braided cable intended for grounding purposes must be used for
the protection device to be effective. In addition to the high
frequency nature of transients, there can be an enormous
amount of current present. Several thousand amps typically
result from applications of the combination wave test in the
ANSI and IEC specification.
Connecting Signal
Grounds
Since a local ground connection is
required at each node implementing shunt type protection, the
consequences of connecting remote grounds together must be
considered. During transient events a high voltage potential
may exist between the remote grounds. Only the impedance in
the wire connecting the grounds limits the current that
results from this voltage potential. The RS-422 and RS-485
specification both recommend using 100 ohm resistors in series
with the signal ground path in order to limit ground currents.
Figure 4.6 illustrates the ground connection recommended in
the specification.
Figure 4.6 Signal Ground Connection between two nodes with
100 ohm resistor
Shunting
Devices
There are two types of shunting devices
to choose from. The least expensive type is single stage,
which usually consists of a single TVS device on each line.
Three stage devices are also available. The first stage of a
three-stage device is a gas discharge tube, which can handle
extremely high currents, but has a high threshold voltage and
is too slow to protect solid state circuits. The second stage
is a small series impedance which limits current and creates a
voltage drop between the first and third stage. The final
stage is a TVS device that is fast enough to protect solid
state devices and brings the clamping voltage down to a safe
level for data circuits.
Combining
Isolation and Shunting
Installing a combination
of both types of protection can offer the highest reliability
in a system. Figures 4.7 and 4.8 illustrate two means of
implementing this level of protection.
Figure 4.7 Isolated node with shunt protection to earth
ground
Figure 4.8 Isolated port with ungrounded shunt
protection
The method shown in Figure 4.7 is recommended, in this case
isolation protects the circuit from any voltage drops in the
earth ground connection. The shunt devices will prevent a
surge from exceeding the breakdown voltage of the isolators as
well as handling any differential surges on the cable. Figure
4.8 illustrates a method recommended for cases where there is
no way to make an earth ground connection. Here, the shunt
device’s function is to protect the port from differential
surges, a differential surge will be balanced between
conductors by the shunting device, converted to common mode.
The isolation provides protection from the common mode
transient remaining.
Special
Consideration for Fault Conditions
Data systems
that could be exposed to short circuits to power conductors
require an extra measure of protection. In these cases its
recommended to add a fuse type device in addition to shunting
type suppression, as shown in Figure 4.9. When a short circuit
occurs, the shunt suppression will begin conducting, but
shunting by itself cannot withstand the steady state currents
of this type of surge. A small enough fuse value should be
chosen so that the fuse will open before the shunt device is
damaged. A typical fuse value is 125 mA.
Figure 4.9 Fused port protection
Choosing the right
protection for your system
While it is hard to
predict what type and level of isolation is correct for a
system, an educated guess should be made based on the
electrical environment, physical conditions and cost of
failures in downtime and repair costs. Systems connected
between two power sources, such as building to building,
office to factory floor, or any system covering long distances
should require some level of transient protection. Table 4.1
is a comparison of transient protection techniques.
| Optical Isolation |
Shunting |
| Requires no ground
reference |
Must have low impedence ground
path |
| Adds no loading to data
lines |
Presents additional capacitive
loading to data lines |
| Higher complexity |
Lower complexity, uses passive
components |
| Effective on common mode
transients |
Effective on both common and
differential mode transients |
| Not dependent on installation
quality |
Can be improperly installed by
user |
| Requires an external power
source |
No power required |
| Not affected by long term or
continuous transients |
Subject to damage by long duration
transients |
Table 4.1 Comparison of
Protection Techniques